Nonadiabatic ab initio molecular dynamics of photoisomerization in bridged azobenzene

Wavelength-dependent photoisomerization of trans-4,4'-azopyridine: Nonadiabatic dynamics simulation

The trans-cis photoisomerization processes of 4,4'-azopyridine upon S1 and S2 excitations have been investigated by nonadiabatic dynamics simulations based on multi-reference CASSCF calculations. 119 sampling trajectories were simulated starting from the trans form excited to the S1 (S2) state and the cis-isomer quantum yield is evaluated to be (3 ± 2)% ((18 ± 4)%), which is qualitatively in agreement with the recent experimental results in ethanol. We found that rotation around the central N-N bond accompanied by the N-N-C symmetrical bending vibrations is the main mechanism in photoisomerization of the target molecule excited to the S1 and S2 states. Upon S1 excitation, S1-S0 transition occurs earlier along the C-N-N-C torsional coordinate, leading to a low cis-isomer quantum yield. Upon S2 excitation, half of the simulated trajectories are trapped in a potential well on the S2 state, from which the twisted conical intersections are more easily reached in the internal conversion, resulting in a higher cis-isomer quantum yield.

Excited-state dynamics of 3-hydroxychromone in gas phase

In recent years, 3-hydroxychromone (3-HC) and its derivatives have attracted much interest for their applications as molecular photoswitches and fluorescent probes. A clear understanding of their excited-state dynamics is essential for their applications and further development of new functional 3-HC derivatives. However, the deactivation mechanism of the photoexcited 3-HC family is still puzzling as their spectral properties are sensitive to the surrounding medium and substituents. The excited-state relaxation channels of 3-HC have been a matter of intense debate. In the current work, we thoroughly investigated the excited-state decay process of the 3-HC system in the gas phase using high-level electronic structure calculations and on-the-fly excited-state dynamic simulations intending to provide insight into the intrinsic photochemical properties of the 3-HC system. A new deactivation mechanism is proposed in the gas phase, which is different from that in solvents. The excited-state intramolecular proton transfer (ESIPT) process that occurs in solutions is not preferred in the gas phase due to the existence of a sizable energy barrier (∼0.8 eV), and thus, no dual fluorescence is found. On the contrary, the non-radiative decay process is the dominant decay channel, which is driven by photoisomerization combined with ring-puckering and ring-opening processes. The results coincide with the observations of an experiment performed in a supersonic jet by Itoh (M. Itoh, Pure Appl. Chem., 1993, 65(8), 1629-1634). The current work indicates that the solution environment plays an important role in regulating the excited-state dynamic behaviour of the 3-HC system. This study thus provides theoretical guidance for the rational design and improvement of the photochemical properties of the 3-HC system and paves the way for further investigation into its photochemical properties in complex environments.

Theoretical Investigation on the Reversible Photoswitch Mechanism of Benzylidene-Oxazolone System

The design and application of molecular photoswitches have attracted much attention. Herein, we performed a detailed computational study on the photoswitch benzylidene-oxazolone system based on static electronic structure calculations and on-the-fly excited-state dynamic simulations. For the Z and E isomer, we located six and four minimum energy conical intersections (MECIs) between the first excited state (S1) and the ground state (S0), respectively. Among them, the relaxation pathway driven by ring-puckering motion is the most competitive channel with the photoisomeization process, leading to the low photoisomerization quantum yield. In the dynamic simulations, about 88 % and 66 % trajectories decay from S1 to S0 for Z and E isomer, respectively, within the total simulation time of ~2 ps. The photoisomeization quantum yields obtained in our study (0.20 for Z→E and 0.12 for E→Z) agree well with the experimental measured values (0.25 and 0.11), even though the number of trajectories is limited to 50. Our study sheds light on the complexity of the benzylidene-oxazolone system 's deactivation process and the competitive mechanisms among different reaction channels, which provides theoretical guidance for further design and development of benzylidene-oxazolone based molecular photoswitches.

Acid Violet 3: A Base-Activated Water-Soluble Photoswitch

Acidic azo dyes are widely used for their vibrant colors. However, if their photophysics were better understood and controllable, they could be integrated into many more applications such as photosensing, photomedicine, and nonlinear optics. Here, the proton-controlled photophysics of a widely used acid, hydrazo dye, acid violet 3 (AV3) is explored. Density functional theory is used to predict the ground- and excited-state potential energy surfaces, and the proposed photoisomerization mechanism is confirmed with spectroscopic experiments. The ground-state and first two excited-state surfaces of the three readily accessible protonation states, AV3-H, AV3, and AV3+H, are investigated along both the dihedral rotation and inversion coordinates. The deprotonated AV3-H undergoes photoisomerization with blue light (λex = 453 nm) through a dihedral rotation mechanism. Upon the formation of the cis-isomer, the reversion of AV3-H is predicted to occur through a mixed rotational and inversion mechanism. In contrast, AV3 and its protonated form, AV3+H, do not undergo photoisomerization because there is no driving force for either the rotation or inversion of the azo bond in the excited state. In addition, when the azo bond is acidic, the ground-state dihedral rotation reversion mechanism barrier is lower. The mechanistic insights gained here through the combination of theory and experiment provide a roadmap to control the reactivity of AV3 across 11 orders of magnitude of proton concentration, making them interesting candidates for a range of pharmaceuticals.

Revealing the excited-state dynamics of cytidine and the role of excited-state proton transfer process

The high photostability of DNAs and RNAs is inextricably related to the photochemical and photophysical properties of their building blocks, nucleobases and nucleosides, which can dissipate the absorbed UV light energy in a harmless manner. The deactivation mechanism of the nucleosides, especially the decay pathways of cytidine (Cyd), has been a matter of intense debate. In the current study, we employ high-level electronic structure calculations combined with excited state non-adiabatic dynamic simulations to provide a clear picture of the excited state deactivation of Cyd in both gas phase and aqueous solution. In both environments, a barrierless decay path driven by the ring-puckering motion and a relaxation channel with a small energy barrier driven by the elongation motion of CO bond are assigned to <200 fs and sub-picosecond decay time component, respectively. The presence of ribose group has a subtle effect on the dynamic behavior of Cyd in gas phase as the ribose-to-base hydrogen/proton transfer process is energetically inaccessible with a sizable energy barrier of about 1.4 eV. However, this energy barrier is significantly reduced in water, especially when an explicit water molecule is present. Therefore, we argue that the long-lived decay channel found in aqueous solution could be assigned to the Cyd-water intermolecular hydrogen/proton transfer process. The present study postulates a novel scenario toward deep understanding the intrinsic photostability of DNAs and RNAs and provides solid evidence to disclose the long history debate of cytidine excited-state decay mechanism, especially for the assignment of experimentally observed time components.

πσ*-Mediated Nonadiabatic Tunneling Dynamics of Thiophenols in S1: The Semiclassical Approaches

The S-H bond tunneling predissociation dynamics of thiophenol and its ortho-substituted derivatives (2-fluorothiophenol, 2-methoxythiophenol, and 2-chlorothiphenol) in S₁ (ππ*) where the H atom tunneling is mediated by the nearby S₂ (πσ*) state (which is repulsive along the S-H bond extension coordinate) have been investigated in a state-specific way using the picosecond time-resolved pump-probe spectroscopy for the jet-cooled molecules. The effects of the specific vibrational mode excitations and the SH/SD substitutions on the S-H(D) bond rupture tunneling dynamics have been interrogated, giving deep insights into the multidimensional aspects of the S₁/S₂ conical intersection, which also shapes the underlying adiabatic tunneling potential energy surfaces (PESs). The semiclassical tunneling rate calculations based on the Wentzel-Kramers-Brillouin (WKB) approximation or Zhu-Nakamura (ZN) theory have been carried out based on the ab initio PESs calculated in the (one, two, or three) reduced dimensions to be compared with the experiment. Though the quantitative experimental results could not be reproduced satisfactorily by the present calculations, the qualitative trends among different molecules in terms of the behavior of the tunneling rate versus the (adiabatic) barrier height or the number of PES dimensions could be rationalized. Most interestingly, the H/D kinetic isotope effect observed in the tunneling rate could be much better explained by the ZN theory compared to the WKB approximation, indicating that the nonadiabatic coupling matrix elements should be invoked for understanding the tunneling dynamics taking place in the proximity of the conical intersection.

Dynamic effects of the bridged structure on the quantum yield of the cis → trans photoisomerization of azobenzene

A nonadiabatic molecular dynamics simulation was performed for the cis → trans photoisomerization of diindane diazocine to determine how its bridged structure results in the highest reported quantum yield for this isomerization. Similar to azobenzene, when diindane diazocine is excited to the S₁ state, it isomerizes to the trans form by a pedal motion of the -NN- moiety passing through the S₁/S₀ conical intersection. However, due to the faster intramolecular vibrational energy redistribution, the excited state lifetime of diindane diazocine is shorter. The bridged structure reduces the degrees of freedom, other than those that drive the isomerization. Therefore, the kinetic energy is selectively distributed to the specific normal mode for the pedal motion of the -NN- moiety, and it is efficiently utilized for the isomerization to the trans form, which is considered a major reason for the increased isomerization yield.

Ultrafast Photoisomerization of N-(2-Methoxybenzylidene)aniline: Nonadiabatic Surface-Hopping Study

We investigated the ultrafast photoisomerization of N-(2-methoxybenzylidene)aniline in the gas phase excited into the second singlet (S₂) state by nonadiabatic surface-hopping dynamics calculations. Two trans isomers (1E and 1E') were taken into consideration in our dynamics simulation. Three conical intersections (CIs) were characterized in the optimization. The CI between S₂ and the first singlet (S₁) states presents a nearly planar structure, while the other two CIs (CI_twist-I and CI_twist-II) between S₁ and the ground (S₀) states show nearly perpendicular geometries. After two trans isomers excited to the S₂ state, the torsion of the C-N bond connected the phenyl group and the stretch of the central bridging bond make the molecule reach CI_planar, and the S₂/S₁ hopping occurs. During the S₁-state dynamics, the molecule moves to a S₁/S₀ CI (CI_twist-I or CI_twist-II) by the rotation of the central bridging bond. The cis isomer is obtained through a barrierless pathway in the S₀ state with the torsion of the three bridging bonds. There is a main channel and an alternative one for the photoisomerization process of both trans isomers. CI_twist-I and CI_twist-II act as S₁/S₀ decay funnels in the main isomerization channels of 1E and 1E' isomers, respectively, and the photochemical processes of 1E and 1E' lead to different cis isomers.

Bridged Azobenzene Enables Dynamic Control of Through-Space Charge Transfer for Photochemical Conversion

Through-space charge transfer (TSCT) has become a thriving strategy of modulating photogenerated charges in organic photoresponsive molecular systems for potential applications in luminescence, optoelectronics, and photochemical conversion. Yet fixed configuration between electron donor (D) and acceptor (A) is disadvantageous to mitigate charge recombination undermining their performances. By carrying out first-principle simulations, we proposed a protocol enabling dynamic control of TSCT within a D-A system by use of a bridged azobenzene (BAB), whose configuration is self-adaptive upon photoexcitation. While the Z-isomer of BAB facilitates π-π stacking of D-A pair with designated frontier orbital alignment to ensure TSCT, the E-isomer of BAB breaks that stacking and restrains charge recombination. Further, as a CO₂ molecule is weakly bound to the anionic acceptor, the former goes bent as a result of charge transfer from the latter, suggesting a path for photodriven CO₂ reduction aided by such a donor-switch-acceptor system. Our proof-of-concept study shows the potential of using specific photoswitch to adaptively steer spatial electron transfer within stacked π systems toward photochemical conversion.

Position of the Benzene Ring Substituent Regulates the Excited-State Deactivation Process of the Benzyluracil Systems

A systematic theoretical study of the regulating effect of the substituent position on the photoinduced deactivation process of the benzyluracil systems has been performed based on the high-level static electronic structure calculations and on-the-fly full-dimensional excited-state dynamics simulations. Similarities and differences coexist for the two systems by comparative studies on the photoinduced deactivation process of the 5-benzyluracil (5-BU) and 6-benzyluracil (6-BU) systems. They both obey an S₂ → S₁ → S₀ two-step decay pattern, and the decay coordinates of the S₂ → S₁ and S₁ → S₀ processes are mainly driven by the elongation of the bridging bond and the out-of-plane ring deformation motion, respectively. However, the puckering motion occurring at the C2 atom in the uracil fragment dominates the decay pathway of the 5-BU system. On the contrary, the puckering motion at the C5 atom in the benzene fragment mainly drives the decay coordinate of the 6-BU system. Therefore, the substituent position could play significant roles in the deactivation process of the benzyluracil systems. Moreover, the S₁ → S₀ decay process of the 6-BU system consists of five pathways, possessing a more complex deactivation picture than the 5-BU system. The fitted time scale of the puckering motion is compatible with the experimentally observed lifetimes. This work provides a fundamental understanding of the photophysical and photochemical properties of the benzyluracil systems and can give rational suggestions to further design or regulate the bionic molecular systems.

Tuning the Thermal Stability and Photoisomerization of Azoheteroarenes through Macrocycle Strain*

Azobenzene and its derivatives are one of the most widespread molecular scaffolds used in a range of modern applications, as well as in fundamental research. After photoexcitation, azo-based photoswitches revert back to the most stable isomer on a timescale ( t 1 / 2 ) that determines the range of potential applications. Attempts to bring t 1 / 2 to extreme values prompted the development of azobenzene and azoheteroarene derivatives that either rebalance the E- and Z-isomer stabilities, or exploit unconventional thermal isomerization mechanisms. In the former case, one successful strategy has been the creation of macrocycle strain, which tends to impact the E/Z stability asymmetrically, and thus significantly modifyt 1 / 2 . On the bright side, bridged derivatives have shown an improved optical switching owing to the higher quantum yields and absence of degradation. However, in most (if not all) cases, bridged derivatives display a reversed thermal stability (more stable Z-isomer), and smaller t 1 / 2 than the acyclic counterparts, which restricts their potential interest to applications requiring a fast forward and backwards switch. In this paper, the impact of alkyl bridges on the thermal stability of phenyl-azoheteroarenes is investigated by using computational methods, and it is revealed that it is indeed possible to combine such improved photoswitching characteristics while preserving the regular thermal stability (more stable E-isomer), and increased t 1 / 2 values under the appropriate connectivity and bridge length.

New insight into the photoinduced wavelength dependent decay mechanisms of the ferulic acid system on the excited states

The ferulic acid (FA) is a kind of phenolic acid widely exists in nature plants. Apart from its medicinal values, the FA is also widely applied in cosmetic industry. Recently, it was found to have potential applications in commercial sunscreens for its strong photostability and photoprotection property from harmful UV rays. Such excellent property lies in the ultrafast decay process of the FA system when exposure to the UV light, but the underlying detailed relaxation pathway is still less clear-cut. In the current work, high-level ab initio electronic structure calculations and on-the-fly surface hopping dynamics simulations were employed to explore the photoinduced decay mechanism of the FA system both on the S₁ and S₃ states in the gas phase. The results provide a reasonable explanation for the wavelength dependent decay patterns of FA system. The S₁ state decay pathway is driven by a re-emission process to dissipate excess energy. While for the S₃ state deactivation process, the pathway is dominated by a non-adiabatic process driven by the internal conversion process through the conical intersection regions. A S₃-S₁-S₀ two step decay pattern is proposed, and the pathways are mainly driven by a puckering distortion motion of the aromatic ring and a twisting motion around the bridging double bond. The calculation results contribute to a better understanding of detailed dynamics behavior of the FA deactivation process, and provide theoretical guidance for further design of efficient and environmentally friendly sunscreens.

Machine learning enables long time scale molecular photodynamics simulations

Photo-induced processes are fundamental in nature but accurate simulations of their dynamics are seriously limited by the cost of the underlying quantum chemical calculations, hampering their application for long time scales. Here we introduce a method based on machine learning to overcome this bottleneck and enable accurate photodynamics on nanosecond time scales, which are otherwise out of reach with contemporary approaches. Instead of expensive quantum chemistry during molecular dynamics simulations, we use deep neural networks to learn the relationship between a molecular geometry and its high-dimensional electronic properties. As an example, the time evolution of the methylenimmonium cation for one nanosecond is used to demonstrate that machine learning algorithms can outperform standard excited-state molecular dynamics approaches in their computational efficiency while delivering the same accuracy.

Volume-conserving photoisomerization of a nonplanar GFP chromophore derivative: Nonadiabatic dynamics simulation

Nonadiabatic dynamics of a nonplanar green fluorescent protein (GFP) chromophore derivative was examined theoretically by trajectory surface hopping approach at the CASSCF level based on Zhu-Nakamura theory. The geometry optimizations show that there are four ground-state minima and four conical intersections between the ground (S₀) and first excited (S₁) states. Four S₁-state minima were found at a perpendicularly twisted conformation around the imidazolinone-bridged bond. Upon excitation to S₁ state, the main decay pathways of four isomers involve different S₀/S₁ potential energy surface conical intersections. The dominant excited-state relaxation mechanism of this GFP chromophore derivative is the twists of two bridging bonds and the methyl group in the bridge combined with the pyramidalization character of the central carbon atom. Further twists of two bridging bonds and the methyl group occur sequentially in the S₀ state. It is worth to mention that the special volume-conserving motion of this molecule is attributed to twists of two bridging bonds in the same direction during the whole photoisomerization processes. The theoretical investigation presented herein provides important insights into the volume-conserving photoisomerization mechanisms of a nonplanar GFP chromophore derivative. We believe that the present work can benefit the design of the photochromic molecule devices in confined media.

Azobenzene-based solar thermal fuels: design, properties, and applications

Development of renewable energy technologies has been a significant area of research amongst scientists with the aim of attaining a sustainable world society. Solar thermal fuels that can capture, convert, store, and release solar energy in the form of heat through reversible photoisomerization of molecular photoswitches such as azobenzene derivatives are currently in the limelight of research. Herein, we provide a state-of-the-art account on the recent advancements in solar thermal fuels based on azobenzene photoswitches. We begin with an overview on the importance of azobenzene-based solar thermal fuels and their fundamentals. Then, we highlight the recent advances in diverse azobenzene materials for solar thermal fuels such as pure azobenzene derivatives, nanocarbon-templated azobenzene, and polymer-templated azobenzene. The basic design concepts of these advanced solar energy storage materials are discussed, and their promising applications are highlighted. We then introduce the recent endeavors in the molecular design of azobenzene derivatives toward efficient solar thermal fuels, and conclude with new perspectives on the future scope, opportunities and challenges. It is expected that continuous pioneering research involving scientists and engineers from diverse technological backgrounds could trigger the rapid advancement of this important interdisciplinary field, which embraces chemistry, physics, engineering, nanoscience, nanotechnology, materials science, polymer science, etc.

Unraveling Photodimerization of Cyclohexasilane from Molecular Dynamics Studies

Photoinduced reactions of a pair of cyclohexasilane (CHS) monomers are explored by time-dependent excited-state molecular dynamics (TDESMD) calculations. In TDESMD trajectories, one observes vivid reaction events including dimerization and fragmentation. A general reaction pathway is identified as (i) ring-opening formation of a dimer, (ii) rearrangement induced by bond breaking, and (iii) decomposition through the elimination of small fragments. The identified pathway supports the chemistry proposed for the fabrication of silicon-based materials using CHS as a precursor. In addition, we find dimers have smaller HOMO-LUMO gaps and exhibit a red shift and line-width broadening in the computed photoluminescence spectra compared with a pair of CHS monomers.

The photoinduced isomerization mechanism of the 2-(1-(methylimino)methyl)-6-chlorophenol (SMAC): Nonadiabatic surface hopping dynamics simulations

The photophysical properties of the Schiff base family are crucial for their applications such as molecular switches and molecular memories. However, it was found that the photophysical behavior is not uniform for all Schiff base molecules, which shows a significant substituent dependent property. In this article, we studied the photoisomerization mechanism of one Schiff base chlorosubstituted derivative 2-(1-(methylimino)methyl)-6-chlorophenol by employing geometrical optimization, energy profiles scanning, and on-the-fly dynamical simulations. Three types of minimum energy conical intersections were located on the S₁/S₀ crossing seam, with two characterized by twisting motion of the C=N bond and one featured with the excited state intramolecular proton transfer process and then twisting motion around the C=C bond [excited-state intramolecular proton transfer process (ESIPT)-then-twisting]. By a combination of the dynamics simulation results with the energy profiles scanned along with the ESIPT coordinate, it was found that the photophysical property of the targeted molecule is different from that of most Schiff base members, which prefer to decay by a twisting motion around the C=N bridge bond rather than the ESIPT-then-twisting channel. The minor ESIPT channel is probably governed by a tunneling mechanism. The proposed deactivation mechanism can provide a reasonable explanation for the observations in the experiment and would provide fundamental indications for further design of new and efficient photochromic products.

Nonadiabatic dynamics simulation of photoisomerization mechanism of the second stablest isomer of N-salicilydenemethylfurylamine

The photoisomerization processes of the second stablest isomer in the aromatic Schiff base, N-salicilydenemethylfurylamine, in the gas phase have been studied by static electronic structure calculations and surface-hopping dynamics simulations based on the Zhu-Nakamura theory. Various stable structures are obtained in the optimization because of different orientations of methyl-furyl part with respect to the salicylaldimine part and different orientations of hydroxy group with respect to the benzene ring. Upon photoexcitation into the first excited state, bond isomerization in the salicylaldimine part is completely suppressed until the strong excited-state hydrogen bond is broken. The decay pathway involves two excited-state minima, one in cis-enol form and the other in cis-keto form. After the excited-state proton transfer, twists of bonds lead to a conical intersection between the ground and excited states. After internal conversion around a conical intersection, the molecule is stabilized in cis- or trans-keto form. If the reverse hydrogen transfer process occurs in the ground state, the molecule will finally end up in the cis-enol region. The cis-keto and trans-keto isomers are observed as photoproducts. According to our full-dimensional nonadiabatic dynamics simulations, we find the excited-state intramolecular proton transfer and torsions of three single bonds in the chain to be responsible for photoisomerization of the second stablest isomer of N-salicilydenemethylfurylamine.

Strain, switching and fluorescence behavior of a nine-membered cyclic azobenzene

This work defines the smallest ring size for obtaining the trans form of cyclic azobenzene as the thermally stable form.

Multi-state nonadiabatic deactivation mechanism of coumarin revealed by ab initio on-the-fly trajectory surface hopping dynamic simulation

An on-the-fly trajectory surface hopping dynamic simulation has been performed for revealing the multi-state nonadiabatic deactivation mechanism of coumarin.

Chiral conversion and periodical decay in bridged-azobenzene photoisomerization: an ab initio on-the-fly nonadiabatic dynamics simulation

On-the-fly trajectory surface hopping dynamics simulations on the cis ↔ trans photoisomerization mechanisms of bridged-azobenzene upon S₁ excitation at the CASSCF level.

Development of semiclassical molecular dynamics simulation method

Various quantum mechanical effects such as nonadiabatic transitions, quantum mechanical tunneling and coherence play crucial roles in a variety of chemical and biological systems.

Non-adiabatic dynamics simulation exploration of the wavelength-dependent photoinduced relaxation mechanism of trans-N-1-methyl-2-(tolylazo) imidazole in the gas phase

A comprehensive picture of the photoinduced non-adiabatic relaxation dynamics of trans-N-1-methyl-2-(tolylazo) imidazole (trans-MTAI) in different electronic excited states has been revealed using the on-the-fly surface hopping method at the ab initio CASSCF level.

Theoretical investigation on remote-control photocycloreversion of dithienylethene driven by azobenzene chromophores

When adding two azobenzene chromophores on a dithienylethene molecule, the irradiation at 450 nm can effectively arouse the cycloreversion reaction of the ring-closed dithienylethene. We investigated the frontier molecular orbitals and absorption properties of such series of molecules and inferred the mechanism of the ring-opening reaction. The 450 nm light sensed by the side azobenzene groups can excite the whole molecule to a high excited state, and through electronic transition and energy transfer the active electron may centre on the dithienylethene ring part, then the ring-opening happens. While for the ring-open form, the energy of the 450 nm light is not high enough to promote the electron to the exact molecular orbital occupying the central dithienylethene ring, so it cannot cause the ring-closing reaction.

Photoisomerization mechanism of 1,1'-dimethyl-2,2'-pyridocyanine in the gas phase and in solution

The trans→cis and cis→trans photoisomerization mechanisms of 1,1'-dimethyl-2,2'-pyridocyanine have been investigated theoretically in the gas phase and in methanol. Two-dimensional potential energy surfaces were computed for the ground and first excited singlet states of the isolated molecule using complete active space self-consistent field method. Our computations suggest that the torsion around the central C-C bonds with carbon-out-of-plane motion is the preferred photoisomerization mechanism. In the gas phase, conical intersections were found near the minima of excited state. The excited-state decay follows a barrierless minimum-energy pathway before the molecule moves to the excited-state global minimum (minS1) and the system relaxes to the ground state through a conical intersection. In methanol, the system would first reach a stationary structure of C2 symmetry after the trans form is electronically excited. Solvent polarity effects were investigated in chloroform, dichloromethane, 1-propanol, ethanol, methanol, and water. There is a significant barrier between the stationary structure of C2 symmetry and minS1 in the excited state in high polarity solvents. Thus, Me-1122P has a much longer lifetime of the excited state in solvents of high polarity.

Trajectory-based nonadiabatic molecular dynamics without calculating nonadiabatic coupling in the avoided crossing case: trans↔cis photoisomerization in azobenzene

We develop a novel method to simulate analytical nonadiabatic switching probability based on effective coupling and effective collision energy by using only electronic adiabatic potential energy surfaces and its gradients in the case of avoided crossing types of nonadiabatic transitions. In addition, the present method can keep the same time step for computing both on-the-fly trajectory and nonadiabatic transitions accurately. The present method is most useful for localized nonadiabatic transitions induced by conical intersection. We employ the on-the-fly surface hopping algorithm with an ab initio quantum chemistry calculation to demonstrate a dynamic simulation for photoisomerization in azobenzene. Simulated quantum yield and lifetime converge to 0.39 and 53 femtosecond, respectively (0.33 and 0.81 picosecond) for cis-to-trans (trans-to-cis) photoisomerization with up to 800 (600) sampling trajectories. The present results agree well with those of the experiment, as well as results simulated with use of nonadiabatic coupling within Tully's fewest switching method. The present trajectory-based nonadiabatic molecular dynamics free from nonadiabatic coupling greatly enhances the simulation power of molecular dynamics for large complex chemical systems.